Line protector

ABSTRACT

A line protector for communications circuits is of a type that comprises a three electrode gas tube wherein two of the electrodes are connected respectively to the line pair and the third electrode is connected to ground. In the event of sustained or extensive overvoltages that cause overheating in the gas tube, damage thereto is prevented by a mechanism that includes a spring-biased contactor which connects both lines of the pair directly to ground. This mechanism has telescoping parts that are held by a solder film therebetween to keep the contactor normally out of its line-grounding position except when the gas tube is heated to the point where it causes the solder film to melt. The mechanism is repositionable following its operation and the resolidification of the solder film so that the protector can be reset for further use without installation of additional parts.

United States Patent [191 Klayum et a1.

1 1 LINE PROTECTOR [75] inventors: Milton A. Klayum, Itasca; Richard E. Nelson, Arlington Heights, both of Ill.

[73] Assignee: Reliable Electric Company, Franklin Park, Ill.

[22] Filed: Nov. 19, 1973 [21] Appl. No.: 417,064

[52] US. Cl. 317/16; 317/40 A; 317/66; 317/615; 337/18 [51] Int. Cl. H0211 3/20 [58] Field of Search 337/152, 199, l6, l8, 17; 317/62, 61.5, 69, 40 A, 51,66,16, 9 R; 313/192 [56] References Cited UNITED STATES PATENTS 3,281,624 10/1966 Wanaselja 317/9 R n] 3,886,408 [4 1 May 27, 1975 Primary ExaminerR. N. Envall, Jr. Attorney, Agent, or F irm-O1son, Trexler, Wolters, Bushnell & Fosse, Ltd.

[57] ABSTRACT A line protector for communications circuits is of a type that comprises a three electrode gas tube wherein two of the electrodes are connected respectively to the line pair and the third electrode is connected to ground. In the event of sustained or extensive overvoltages that cause overheating in the gas tube, damage thereto is prevented by a mechanism that includes a spring-biased contactor which connects both lines of the pair directly to ground. This mechanism has telescoping parts that are held by a solder film therebetween to keep the contactor normally out of its linegrounding position except when the gas tube is heated to the point where it causes the solder film to melt. The mechanism is repositionable following its operation and the resolidification of the solder film so that the protector can be reset for further use without installation of additional parts.

10 Claims, 9 Drawing Figures ll ll will.

LINE PROTECTOR BACKGROUND OF THE INVENTION This invention relates to improvements in protective devices for electrical lines, more particularly for telephone and like communications lines.

It is a common practice to protect telephone and similar communications lines from overvoltage conditions which might tend to injure personnel or damage equipment connected to such lines. One general form of protector known in the art is a unit that utilizes a three electrode gas-filled tube and a mounting for the same such that two of the electrodes are connected respectively to the line pair while the third electrode is connected to ground. Overvoltages of short duration from either line of the pair cause an are within the tube to the third electrode so that these overvoltages are grounded and the unit is automatically restored for continued operation. Sometimes, however, the overvoltage fault is of sufficient magnitude or is of such duration that an overcurrent condition exists causing the gas tube to be overheated and damaged beyond use. When this condition occurs the gas tube no longer adequately serves as a line protector and damage to equipment or personnel may result.

Accordingly, the known types of three electrode protector units embody an arrangement for grounding of the lines in the event that the gas tube becomes overheated. Such arrangements utilize a grounding bar or contactor that is spring biased toward its linegrounding position but is normally restrained from such position by one or more solder pellets that are in thermal contact with the grounding or third electrode of the gas tube. When the tube overheats the solder pellet or pellets melt, whereupon the spring urges the grounding bar to its line-grounding position whereby the lines of the pair are directly grounded, bypassing the tube. The unit is restored to normal operation by, among other things, the reinstallation of a new solder pellet or pellets, as the case may be.

The use of solder pellets in mechanisms of the foregoing type is not entirely satisfactory. After fusing or melting, the solder tends to be lodged in the mechanism as small pieces which may render restoration of the unit impossible without extensive repair. Furthermore, during an overload condition the melting solder may lodge itself in the mechanism such that the grounding bar does not ground both pair of conductors of the line. Also when the solder melts it may become so dispersed in the unit that when the solder ultimately solidifies, it may cause an unwanted short circuit. If that occurs the solder must be cleaned out carefully and it may even by necessary to rewire the device.

Another problem with solder pellets lies in the fact that for ease in handling they must be of substantial mass and they must be inventoried and carried by, or at least available to, repair personnel. Since these solder pellets are of a particular configuration and composition for use with a particular unit, the fabrication of a solder pellet by repair personnel is considered impractical.

A still further disadvantage of solder pellets lies in the fact that a substantial length of time is required for fusing because the fusing time is controlled not only by the mass of the pellet but also by the area of the pellet that engages the gas tube. Such area is restricted by the relatively limited area of engagement available in the usual type of three electrode gas tube.

A still further problem with solder pellets lies in the fact that they are subject to cold flow conditions. The solder pellet is generally under compression from the spring force that is applied to the grounding bar, and this pressure may cause cold flow of the solder pellet and possibly premature grounding of the lines when no fault is present.

OBJECTS AND SUMMARY OF THE INVENTION In general, an object of the present invention is to provide a line protector of the foregoing general type but which overcomes the aforesaid difficulties thereof.

It is a further object of this invention to provide a protector of a type stated in which the solder is in the form of a film that retains telescopic parts of the mechanism together, and wherein these telescoping parts are repositionable to reset or restore the unit to normal operating conditions following a line fault condition in which the lines have become grounded by the grounding bar or contactor.

It is a still further object of this invention to provide a protector of the type stated in which the film of solder is ofa relatively small mass to increase the melting time of the solder, and thereby increase the time response of the unit to a line fault condition.

Another object of this invention is to provide a protector of the type stated in which solder pellets need not be inventoried by users of the protector.

In accordance with the foregoing objects, an embodiment of the invention comprises a mounting for a gas tube having three electrodes which are electrically insulated from each other and wherein there is an arc gap within the tube between each of first and second electrodes and a third electrode. The mounting includes three terminals electrically insulated from each other and means for removably supporting the gas tube with the three electrodes in electrical contact with each of the terminals respectively. The arrangement also in cludes a grounding bar or contactor, resilient means urging the contactor from a normal location out of electrical contact with the terminal associated with the first and second electrodes to a location of electrical contact with the three terminals, and mechanism preventing the electrical contact of the contactor with the terminals associated with the first and second electrodes except upon overheating of the gas tube. The mechanism is characterized by meltable means, such as a solder film, for heat reception from the gas tube, and cooperating parts rigidly held in their relative positions by the solder film except upon overheating of the tube but relatively movable to other relative positions upon melting of the solder film due to said overheating. Furthermore, the cooperating parts are repositionable upon solidification of the solder film to reset the contactor means in its normal location.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a side elevational view of a protector unit constructed in accordance with and embodying the present invention;

FIG. 2 is an elevational view thereof as seen from the left hand end of FIG. 1;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 1 but with certain of the parts in elevation;

FIG. 4 is an enlargement of a portion of the arrangement of FIG. 3 but with parts of the mechanism that are in elevation in FIG. 3 being shown in section;

FIG. 5 is a fragmentary perspective of a part of the center clip and the contactor;

FIG. 6 is a fragmentary enlarged view similar to FIG. I partly in section and showing the condition of the mechanism after the fusible solder film has melted as a result of an overcurrent condition on one or more of the lines;

FIG. 7 is a fragmentary sectional view similar to FIG. 3 but showing the modified form of protector;

FIG. 8 is a fragmentary perspective view of the center clip of FIG. 7', and

FIG. 9 is a sectional view taken along line 99 of FIG. 7.

DETAILED DESCRIPTION Referring now in more detail to the drawing there is shown a protector 2 having a base 4 formed of suitable insulating material. The base 4 has three spaced apart members extending therethrough and which constitute terminals 6, 8, 10 that are insulated from each other. The terminals 6, 8, 10 may be screw threaded members that serve as binding posts for connections of wires thereto. In a typical installation the terminals 6, 10 are respectively connected to the conductors of a telephone line pair while the center terminal 8 is connected to ground.

Mounted on the base 4 by the terminals 6, 10 are re silient L-shaped, metallic terminal clips l2, 12. The terminal clips l2, 12 are aligned with one another along the base 4 and each includes, at its upper end, a V shaped projection 14. Spaced below the projection 14 each clip 12 also has an inwardly struck finger 16. The base potions of the clips 12 are retained against the base 4 in a known manner as by the heads of the terminals 6, 10, the terminals themselves being held rigidly on the base 4 by nuts 18.

Intermediate the clips 12, 12 is a U-shaped, resilient, metal grounding or center clip 20, the base of which is held against the base 4 by the head on the ground terminal 8. The center clip has opposed sides 22, 22 with inwardly struck flanges 24, 24 the free ends of which are inset within the sides 22, 22, for purposes presently more fully appearing. At their free ends, the sides 22, 22 have generally arcuate segments 26, 26.

Removably mounted in the terminal clips l2, l2 and the center clip 20 is a cylindrical three electrode gas filled tube 28 known construction. Suffice it to say, however, that the gas tube 28 has opposed electrodes 30, 32 (FIG. 6) each coaxial of the gas tube and each being mechanically and electrically connected to respective end caps 38, 38. The end caps 38, 38 are retained in firm contact with the terminal clips l2, 12 between the respective projections 14 and fingers 16. It will be noted from FIG. 2 that the free end of each terminal clip l2, I2 is bifurcated to provide a slot 40 for accomodating a conventional sealing projection that may be on the tube and may extend axially from one of the end caps 38.

The center electrode 42 is a hollow cylindrical mem her in coaxial surrounding relationship with the electrodes 30, 32. The electrodes 30, 32, 42 are separated from each other by insulating spacers 44, 45 such that are gaps within the tube 28 are formed between the opposed electrodes 30, 32 and the central electrode 42.

The arcuate segments 26, 26 on the center clip 20 clinch the center electrode 42 to retain the tube on its mounting and also to provide a metallic path to ground through the center clip 20 and the terminal 8. Thus, when a transient overvoltage condition appears on one or both of the lines being protected there will be an are between one or both of the electrodes 30, 32 and the center electrode 42, which will cause the transient current to flow to ground.

Disposed between the center clip sides 22, 22 is a generally U-shaped grounding bar or contactor 46. The contactor 46 is an elongated metallic strip that extends to regions adjacent to but spaced from the terminal clips l2, 12. A coil compression spring 50 bears against the base portion of the center clip 20 and also against the contactor 46 approximately midway between its opposite ends for biasing the contactor 46 toward the fingers l6, 16 of the terminal clips l2, 12. The contactor 46 may have small cars 47 (FIGS. 4 and 5) on opposite sides of the flanges 24, 24 for positioning the contactor midway between the clips l2, l2.

In the normal operation of the protector, the contactor ends 52, 52 are held in spaced relationship to the fingers l6, 16 as shown in FIG. 1, by a mechanism that is interposed between the center electrode 42 and the contactor 46. This mechanism, best shown in FIGS. 4 and 6, comprises an outer metallic sleeve 54 having arcuate flange portions 56, 56 at its opposite ends. The arcuate flange portion 56 is of such size to provide a substantial area of direct surface-to-surface metallic contact with the center electrode 42 so as to insure good thermal transmission of heat from the center elec trode 42 to the outer sleeve 54. Telescoped within the outer sleeve 54 and cooperating therewith is an inner metallic sleeve 58 which is substantially shorter in length than the outer sleeve 54. The outer and inner sleeves 54, 58 are held rigidly together by a film of meltable material, such as solder 60, that coats the full length of the bore of the inner sleeve 54 and also the full inner circumference thereof. The solder film is, therefore, also bonded to the outer surface of the inner sleeve 58. The solder may be of any suitable type, for instance eutectic solder that melts at 294 F. A pin 62 seats on the contactor 46 and has a small diameter end portion 64 that projects through the contactor 46 and is staked or otherwise permanently secured thereto. The other end of the pin 62 projects axially into the bore of the outer sleeve 54 and has a shoulder 65 that engages the end of the inner sleeve 58. A reduced diameter end 67 of the pin slides into the inner sleeve 58. The pin 62 is preferably made of stainless steel or other material to which the solder will not adhere.

When an overvoltage condition occurs on either of the lines to be protected that causes the gas tube to become heated, this heat will be transmitted from the center electrode 42 to the outer sleeve 54 and therefore to the solder film 60. As the solder melts, the shear force thereon applied by the sping 50 through the pin 62 and inner sleeve 58 will cause the inner sleeve 58 to move axially within the outer sleeve 54 from the position shown in FIG. 4 to that shown in FIG. 6. As a result, the contactor 46 is urged from its normal location (FIGS. 1 and 4) to its line-grounding location shown in FIG. 6 wherein the contactor ends 52 engage the fingers l6, 16 of the terminal clips l2, 12. This immediately grounds both terminals 6, l0 and hence grounds the lines connected thereto. With the excessive current now bypassing the gas tube 28 the latter becomes cooled causing the solder film 60 to resolidify. The inner and outer sleeves 58, 54 will then be rigidly bonded together in the positions shown in FIG. 6.

It should also be noted that when the contactor 46 is shifted to engage the fingers 16, 16 the contactor will be in firm engagement with the flanges 24, 24, This assures that the ground connection through the contactor 46 is through the center clip 20 so that the spring 50 need not be relied upon to carry the current. For this purpose the distance between the flanges 24, 24 will be made slightly smaller than the width of the contactor 46 thereat. The flanges 24, 24 can spring outwardly somewhat so as not to impede, to any significant extent, the movement of the contactor 46.

The protector may be reset for normal operation by repositioning the unit consisting of the inner and outer sleeves 58, 54. For this purpose the gas tube 28 may first be withdrawn from its mounting whereupon the inner and outer sleeves as a unit may be withdrawn from the pin 62, turned end for end, and reinserted onto the pin 62. At this time the contactor 42 is still in engagement with the fingers l6, 16. However, when the gas tube 28 is replaced in the clips l2, 12, 20 pressure is applied through the center electrode 42 to the outer sleeve 54, the inner sleeve 58 and the pin 62 to depress the spring 50, thereby shifting the contactor 42 out of engagement with the fingers 16, 16 to complete resetting of the protector. The retaining forces of the clips l2, 12 20 on the gas tube is greater than the force of the spring 50.

In order to insure that the unit consisting of the inner and outer sleeves 58, 54 will function properly when turned end for end, the length of the inner and outer sleeves and the distance that the contactor 46 moves should be controlled. Thus, the distance from the lower axial end (i.e., at shoulder 65) of the inner sleeve 58 to the lower axial end of the outer sleeve 54 in the normal condition of the protector should be the same as the distance from the upper axial end of the inner sleeve 58 to the upper axial end of the outer sleeve 54 after the protector has operated to ground the lines, as aforesaid. In this way the spacing of the ends 52,52 from the fingers 16, 16 will be the same each time the sleeve unit 58, 54 is turned end for end to reset the protector.

It is also possible to provide a modified form of mounting for a three electrode gas tube, such as shown in FIGS. 7 9, and wherein like reference numerals indicate like parts, previously described. in the arrangement of H65. 7-9, the gas tube 28a is of smaller length than the gas tube 28 and hence the terminal clips 12a, 120 will be correspondingly closer together and the contactor 46a will be shorter in length. The gas tube 28a may have end electrode caps 38a, 38a and a center electrode 420 in the form ofa raised rim. Each arcuate segment 260 on the center clip 20a is shaped with an arcuate recess 67a to accommodate and provide good contact with the center electrode 420. Likewise, the arcuate flange portions 56a of the outer sleeve 54a have arcuate grooves 69 for receiving and making contact with the center electrode 42a. In other respects the protector of FlGS. 7 9 functions in the same manner as does the protector of FIG. I 6.

The invention is claimed as follows:

1. A mounting for a gas tube having three electrodes which are electrically insulated from each other and wherein there is an arc gap within the tube between each of first and second electrodes and a third electrode, said mounting including three terminals electrically insulated from each other, means for removably supporting said gas tube with said three electrodes in electrical contact with each of said terminals respectively, contactor means, resilient means urging said contactor means from a normal location out of electrical contact with the terminals associated with said first and second electrodes to a location of electrical contact with said three terminals, and mechanism preventing said electrical contact of the contactor means with said terminals associated with said first and second electrodes except upon overheating of said gas tube; said mechanism comprising meltable means in the form of a film for heat reception from said gas tube, and cooperating relatively slidable parts rigidly held in fixed positions by said meltable means between said cooperating parts except upon said overheating but relatively slidable upon melting of said meltable means due to said overheating, said parts being repositionable upon solidification of said meltable means and without replacement thereof to reset said contactor means in its normal location, and at least one of said parts being in force-transmitting engagement with said resilient means and imposing a shear force on said meltable means.

2. A mounting for a gas tube having three electrodes which are electrically insulated from each other and wherein there is an arc gap within the tube between each of first and second electrodes and a third electrode, said mounting including three terminals electrically insulated from each other, means for removably supporting said gas tube with said three electrodes in electrical contact with each of said terminals respectively, contactor means, resilient means urging said contactor means from a normal location out of electrical contact with the terminals associated with said first and second electrodes to a location of electrical contact with said three terminals, and mechanism preventing said electrical contact of the contactor means with said terminals associated with said first and second electrodes except upon overheating of said gas tube; said mechanism comprising meltable means for heat reception from said gas tube, and cooperating parts rigidly held in relative positions by said meltable means except upon said overheating but relatively movable to other relative positions upon melting of said meltable means due to said overheating, said parts being repositionable upon solidification of said meltable means to reset said contactor means in its normal location. One of said cooperating parts being telescoped within another of said cooperating parts, said meltable means being radially interposed between those last-mentioned parts, and said resilient means imposing a shear force on said meltable means.

3. A mounting according to claim 2 in which said one part is movable axially within the other part, upon said melting, an amount such that said parts are repositionable, upon said solidification, by removal of said parts as a unit from said mounting and reinstallation of the unit in a position that is inverted from its previous position.

4. A mounting according to claim 3 further having a pin mounted on said contact means and engaging the inner of the telescoping parts to push the same relative to the outer telescoping part upon said melting.

S. A mounting according to claim 1 having said gas tube mounted thereon by said means for removably supporting the same, and said cooperating parts being in heat conductive contact with said third electrode,

6. A mounting according to claim 2 having said gas tube mounted thereon by said means for removably supporting the same, and the outer of said cooperating parts being in heat conductive contact with said third electrode.

7. A protector comprising three electrode gas tube having two opposed electrodes and a third electrode surrounding the two opposed electrodes and forming an arc gap with each, a base having terminals thereon that are electrically insulated from each other, means for mounting said gas tube on said base such that the electrodes are each respectively connected to one of said terminals for connecting said opposed electrodes to lines to be protected and for connecting said third electrode to ground, contactor means, resilient means urging said contactor means from a normal location out of electrical contact with the terminals associated with said opposed electrodes to a location of electrical contact with all of said terminals thereby to connect the terminals associated with said opposed electrodes to ground and mechanism preventing electrical contact of said contactor means with said last-mentioned terminals except upon overheating of said gas tube; said mechanism having cooperating parts rigidly held in relative positions by meltable means between said cooperating parts in thermal contact with said third electrode. said parts being relatively movable upon melting of said meltable means due to said overheating and being re positionable upon solidification of said meltable means without replacement thereof to reset said contactor means in its normal location said meltable means being a film of material upon which a shear force is imposed by at least one of said cooperating parts 8. A protector according to claim 7 in which said cooperating parts are repositionabel by repositionable as a unit to position that is turned from its previous posi' tion,

9. A protector comprising a three electrode gas tube having two opposed electrodes and a third electrode surrounding the two opposed electrodes and forming an arc gap with each a base having terminals thereon that are electrically insulated from each other. means for mounting said gas tube on said base such that the electrodes are each respectively connected to one of said terminals for connecting said opposed electrodes to lines to be protected and for connecting said third electrode to ground, contactor means, resilient means urging said contactor means from a normal location out of electrical contact with the terminals associated with said opposed electrodes to a location of electrical contact with all of said terminals thereby to connect the terminals associated with said opposed electrodes to ground. and mechanism preventing electrical contact of said contactor means with said last-mentioned termi nals except upon overheating of said gas tube; said mechanism having cooperating parts rigidly held in rel ative positions by meltable means in thermal contact with said third electrode, said parts being relatively movable upon melting of said meltable means due to said overheating and being repositionable upon solidification of said meltable means to reset said contactor means in its normal location, said cooperating parts being telescoped and with the meltable material being therebetween.

10. A protector according to claim 7 having a clip for engaging said third electrode, said clip having an arcuate recess that receives said third electrode. 

1. A mounting for a gas tube having three electrodes which are electrically insulated from each other and wherein there is an arc gap within the tube between each of first and second electrodes and a third electrode, said mounting including three terminals electrically insulated from each other, means for removably supporting said gas tube with said three electrodes in electrical contact with each of said terminals respectively, contactor means, resilient means urging said contactor means from a normal location out of electrical contact with the terminals associated with said first and second electrodes to a location of electrical contact with said three terminals, and mechanism preventing said electrical contact of the contactor means with said terminals associated with said first and second electrodes except upon overheating of said gas tube; said mechanism comprising meltable means in the form of a film for heat reception from said gas tube, and cooperating relatively slidable parts rigidly held in fixed positions by said meltable means between said cooperating parts except upon said overheating but relatively slidable upon melting of said meltable means due to said overheating, said parts being repositionable upon solidification of said meltable means and without replacement thereof to reset said contactor means in its normal location, and at least one of said parts being in force-transmitting engagement with said resilient means and imposing a shear force on said meltable means.
 2. A mounting for a gas tube having three electrodes which are electrically insulated from each other and wherein there is an arc gap within the tube between each of first and second electrodes and a third electrode, said mounting including three terminals electrically insulated from each other, means for removably supporting said gas tube with said three electrodes in electrical contact with each of said terminals respectively, contactor means, resilient means urging said contactor means from a normal location out of electrical contact with the terminals associated with said first and second electrodes to a location of electrical contact with said three terminals, and mechanism preventing said electrical contact of the contactor means with said terminals associated with said first and second electrodes except upon overheating of said gas tube; said mechanism comprising meltable means for heat reception from said gas tube, and cooperating parts rigidly held in relative positions by said meltable means except upon said overheating but relatively movable to other relative positions upon melting of said meltable means due to said overheating, said parts being repositionable upon solidification of said meltable means to reset said contactor means in its normal location. One of said cooperating parts being telescoped within another of said cooperating parts, said meltable means being radially interposed between those last-mentioned parts, and said resilient means imposing a shear force on said meltable means.
 3. A mounting according to claim 2 in which said one part is movable axially within the other part, upon said melting, an amount such that said parts are repositionable, upon said solidification, by removal of said parts as a unit from said mounting and reinstallation of the unit in a position that is inverted from its previous position.
 4. A mounting according to claim 3 further having a pin mounted on said contact means and engaging the inner of the telescoping parts to push the same relative to the outer telescoping part upon said melting.
 5. A mounting according to claim 1 having said gas tube mounted thereon by said means for removably supporting the same, and said cooperating parts being in heat conductive contact with said third electrode.
 6. A mounting according to claim 2 having said gas tube mounted thereon by said means for removably supporting the same, and the outer of said cooperating parts being in heat conductive contact with said third electrode.
 7. A protector comprising three electrode gas tube having two opposed electrodes and a third electrode surrounding the two opposed electrodes and forming an arc gap with each, a base having terminals thereon that are electrically insulated from each other, means for mounting said gas tube on said base such that the electrodes are each respectively connected to one of said terminals for connecting said opposed electrodes to lines to be protected and for connecting said third electrode to ground, contactor means, resilient means urging said contactor means from a normal location out of electrical contact with the terminals associated with said opposed electrodes to a location of electrical contact with all of said terminals thereby to connect the terminals associated with said opposed electrodes to ground, and mechanism preventing electrical contact of said contactor means with said last-mentioned terminals except upon overheating of said gas tube; said mechanism having cooperating parts rigidly held in relative positions by meltable means between said cooperating parts in thermal contact with said third electrode, said parts being relatively movable upon melting of said meltable means due to said overheating and being repositionable upon solidification of said meltable means without replacement thereof to reset said contactor means in its normal location, said meltable means being a film of material upon which a shear force is imposed by at least one of said Cooperating parts.
 8. A protector according to claim 7 in which said cooperating parts are repositionabel by repositionable as a unit to position that is turned from its previous position.
 9. A protector comprising a three electrode gas tube having two opposed electrodes and a third electrode surrounding the two opposed electrodes and forming an arc gap with each, a base having terminals thereon that are electrically insulated from each other, means for mounting said gas tube on said base such that the electrodes are each respectively connected to one of said terminals for connecting said opposed electrodes to lines to be protected and for connecting said third electrode to ground, contactor means, resilient means urging said contactor means from a normal location out of electrical contact with the terminals associated with said opposed electrodes to a location of electrical contact with all of said terminals thereby to connect the terminals associated with said opposed electrodes to ground, and mechanism preventing electrical contact of said contactor means with said last-mentioned terminals except upon overheating of said gas tube; said mechanism having cooperating parts rigidly held in relative positions by meltable means in thermal contact with said third electrode, said parts being relatively movable upon melting of said meltable means due to said overheating and being repositionable upon solidification of said meltable means to reset said contactor means in its normal location, said cooperating parts being telescoped and with the meltable material being therebetween.
 10. A protector according to claim 7 having a clip for engaging said third electrode, said clip having an arcuate recess that receives said third electrode. 